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<Paper uid="C86-1088">
  <Title>Definite Noun Phrases and the Semantics of Discourse</Title>
  <Section position="1" start_page="0" end_page="368" type="abstr">
    <SectionTitle>
1. Introduction
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    <Paragraph position="0"> Discourse Representation Theory (DRT), developed by Hans Kamp several years ago (Kamp 1981), belongs, together with Irene Heims narrowly related File Change Semantics (Heim 1982) and Situation Semantics (Barwise/Perry 1983), to a grovp of theoretical approaches which in the early Eighties introduced a dynanfic, context-oriented perspective into the semantics of natural language. This recent development in theoretical semantics indicates a shift of interest towards topics that have been familiar in natural language processing research for the last decade: among others, the inteq~retation of new utterances with respect to a given context, and integration of the utterance information into that context; the step-by-step construction of representations for larger pieces of discourse; the investigation of text coherence phenomena; and the description of referential processes.</Paragraph>
    <Paragraph position="1"> The corn of DRT (and File Change Semantics) is the treatment of indefinite noun phrases as reference establishing terms (as opposed to their standard truth-conditional quantifier analysis, but in accordance with the treatmant of indefinites in NLP research) and definite noun phrases (pronouns as well as full NPs) as anaphoric expressions. It is one of the theoretically most appealing features of these theories that they provide simple unified accounts for all indefinites, and for all definites, respectively. This theoretical simplicity stands however in sharp contrast to the complexity of the process of etablishing reference observed in NLP research, and the variety of phenomena and linguistic levels involved. On the one hand, this contrast is quite natural: As a semantically motivated theow, DRT should not be expected to incorporate every detail of inferencing necessary to come up with an interpretation for a specific utterance in a given context; it can better be thought of as an interface relating theoretical, truth-conditional semantics and the genuinely pragmatic work of text understanding. On the other hand, if DRT is seriously intended to bridge the gap between theoretical linguistics and the NLP approach, it should take into consideration as many factual restrictions on NP reference, and distinctions among subtypes of referential expressions, as is possible in a systematic and descriptive way. Several extensions of the standard system are at work, e.g. for the treatment of plural and temporal anaphora. Little, however, has yet been done to arrive at a closer view of the analysis of (singular) definite noun phrases, once the basic concepts had been established. The only attempt I know about is by Kamp himself, described in Kamp (1983), an unpublished fragment.</Paragraph>
    <Paragraph position="2"> In this talk I will first give a short overview of the basic DRT system, and sketch Kamp's proposal for the treatment of definite noun phrases. Then I will indicate how the basic reference establishing function and the &amp;quot;side-effects &amp;quot;of different types of definite NPs can be described in more detail. In doing this, I will refer to the work about anaphora done in the NLP area (esp. by Barbara Grosz, Candy Sidner, and Bonnie Webber), integrating some of their assnrnptions into the DRT framework, and critically comanenting on some others.</Paragraph>
    <Paragraph position="3">  2. &amp;quot;File Framework of Discourse Representation Theory To illustrate the basic principles of DRT, let us look at the following two-sentence text: (1) (a) John owns a book.</Paragraph>
    <Paragraph position="4"> (b) He reads it.</Paragraph>
    <Paragraph position="5">  In an utterance of (1 a), two discourse referents (DRs) are introduced by the NPs John and the book, and several data concerning these referents are communicated. These facts about (1 a) are represented by the &amp;quot;discourse representation structure'(DRS) K1 under (2 a), which consists of a universe of discourse (U K ) as well as a set of conditions (CONK) on the members of U K,  (2) KI: John (x)  |x owns y |  DRSes are built up by DRS construction rules, which operate on a straightforward phrase-structure analysis of the sentence, and are sensitive to the context of utterance. DRSes for texts are construed by sequentially analysing sentence by sentence, referring to the DRS built up so far as the context of utterance, and extending this DRS by the discourse referents and conditions emerging from the application of the construction rules to the sentence under consideration. Sentence (1 b) requires application of a very simple context-sensitive construction rule, the rule for anaphoric pronouns, which can roughly be formulated as (3):  (3) (i) Add a new DR x to U,~ (K being the current DRS).</Paragraph>
    <Paragraph position="6"> (ii) Replace the anaphoric pronoun in the (syntactic description of) the constituent under consideration by x.</Paragraph>
    <Paragraph position="7"> (iii) Add x = y as a new condition, for some DR y already  contained in UI{.</Paragraph>
    <Paragraph position="8"> The crucial part of this rule is clause (iii) which says that the pronoun should be anaphorically linked to a referent already present in the context of utterance. By applying (3) twice to (1 b), the DRS K1 is extended to K2.</Paragraph>
    <Paragraph position="9"> The author is Heisenberg fellow of the Deutsche Forschungsgemeinschaft.</Paragraph>
    <Paragraph position="11"> The step-by-step construction of a DRS for a given text is the first part of its semantic analysis. The analysis is completed by embedding the DRS representing the text information into a conventional first-order model structure. A proper embedding is a function from 1J/~ to the model universe, assigning real-world objects to DRs in a way that all conditions of the DRS are satisfied.</Paragraph>
    <Paragraph position="12"> A text is true if a proper embedding for its DRS exists. Thus, the embedding provides a truth-conditional background for the DRS formalism. E.g., it indirectly provides the indefinite NP a book in (1 a), the function of which is described as introduction of a new referent on the DRS level, with its usual existential inteq~retation.</Paragraph>
    <Paragraph position="13"> Conditionals, universal quantifiers, negation require a DRS analysis in terms of complex conditions on sub-DRSes. Sentence (5), e.g., is represented by the DRS (6), where the left-band box is the sub-DRS representing the antecedent, and the right-hand box represents the consequent of (5).</Paragraph>
    <Paragraph position="14">  (5) If John owns a book, he reads it.</Paragraph>
    <Paragraph position="16"> Universal sentences like (7) are represented by the same type of complex DRSes.</Paragraph>
    <Paragraph position="17"> (7) Every man who owns a book reads it.</Paragraph>
    <Paragraph position="18"> The embedding rule for =&gt; - conditions requires, roughly speaking, that every proper embedding for the antecedent sub-DRS can be properly extended to the consequent sub-DRS. By this, the indefinite NP a book in (5), which on the DRS-level has identical function with a book in (1 a), i.e. DR introduction, is correctly interpreted as a universal qnantifier.</Paragraph>
    <Paragraph position="19"> The internal structure of DRSes allows one to formulate an important restriction on the use of anaphoric expressions: DRs introduced in sub-DRSes must not be anaphorically accessed from outside. This restriction excludes a continuation of (5) or (7) by a sentence like (8): (8) It has 200 pages.</Paragraph>
    <Paragraph position="20"> Accessible for anaphoric linking in a DRS K is UN itself, the universe of all superordinate DRSes, and, possibly, the universe of the antecedent DRS (if K represents a consequent of a conditional), but nothing else. This restriction seems to be more or less valid for all kinds of anaphoric expressions; i will not discuss problematic cases here (cf., however, Sells 1985). Instead, I will look at the way Kamp further differentiates between different types of anaphoric noun phrases.</Paragraph>
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